The Impact of Russia-Ukraine geopolitical conflict on the air quality and toxicological properties of ambient PM2.5 in Milan, Italy

The geopolitical conflict between Russia and Ukraine has disrupted Europe’s natural gas supplies, driving up gas prices and leading to a shift towards biomass for residential heating during colder months. This study assessed the consequent air quality and toxicological impacts in Milan, Italy, focusing on fine particulate matter (PM2.5, dp < 2.5 μm) emissions. PM2.5 samples were analyzed for their chemical composition and assessed for their oxidative potential using the dithiothreitol (DTT) assay across three periods reflecting residential heating deployment (RHD): pre-RHD, intra-RHD, and post-RHD periods. During the intra-RHD period, PM2.5 levels were significantly higher than those in other periods, with concentrations reaching 57.94 ± 7.57 μg/m3, indicating a deterioration in air quality. Moreover, levoglucosan was 9.2 times higher during the intra-RHD period compared to the pre-RHD period, correlating with elevated levels of elemental carbon (EC) and polycyclic aromatic hydrocarbons (PAHs). These findings were compared with previous local studies before the conflict, underscoring a significant rise in biomass-related emissions. DTT assay levels during the intra-RHD were 2.1 times higher than those observed during the same period in 2022, strongly correlating with biomass burning emissions. Our findings highlight the necessity for policies to mitigate the indirect health effects of increased biomass burning emissions due to the energy crisis triggered by the geopolitical conflict.


Sampling procedure
In this study, meticulous attention was given to the calibration and accuracy of PCISs to ensure the reliability of our data.Standard operating procedures included flow rate verifications and leak tests, ensuring samplers adhered to manufacturer specifications for precise PM2.5 collection, which was detailed in prior studies [1][2][3][4][5][6][7][8][9] .Upon passing acceptance tests, samplers were deployed in the monitoring site, where recalibration and regular performance checks were conducted according to fixed schedules.The quartz filters used for sampling were prebaked, subsequently stored in freezers in Teflon petri dish containers lined with pre-fired aluminum foil, and then equilibrated under controlled conditions to avoid carbon contamination.Additionally, Polonium-210 sources were used for neutralization before weighing with an MT5 microbalance (Mettler-Toledo Inc., Columbus, OH; uncertainty of 1 μg).Moreover, Sterile, solvent-rinsed stainlesssteel tweezers were used for all the work with respect to filters.To maintain samples integrity, our team followed chain-of-custody procedures to maintain sample integrity, including the preparation of filter packs, all carefully labeled and shipped in dry ice to the field site in a sealed cooler to prevent any contamination.The concentrations of collected samples were calculated as a function of sample volumes, calculated from flow rates and sampling durations.Cross-verification of PM2.5 mass concentrations with the Roveda di Sedriano (MI) air monitoring site in Bareggio and maintaining a minimum of 85% successful data capture rate further underscored the robustness of our data collection process.This comprehensive approach to calibration, accuracy, and data handling was pivotal in ensuring the reliability of the chemical and DTT assay analyses conducted at facilities of the DRI and University of Illinois Urbana-Champaign (UIUC), respectively.

Chemical and toxicological analysis
The measurement of OC and EC, as well as the assessment of the volatility of OC fractions, were conducted using a multiwavelength thermal/optical carbon analyzer (Magee Scientific, Berkeley, CA, USA), adhering to the protocols delineated by the interagency monitoring of protected visual environments (IMPROVE_A) 10 .The protocol involved a gradual heating of the quartz filter to specific temperatures for the determination of OC concentrations, followed by a temperature increase for EC level assessment.Additionally, gas chromatography-mass spectrometry instruments (GC-MS, Model 6890N GC/5973 MS detectors, Agilent, Santa Clara, CA, USA) were utilized to measure levoglucosan and PAH 11 .Filters were extracted with methanol and methylene chloride using sonication and then concentrated using a rotary evaporator and then a nitrogen evaporator.The extract was derivatized using diazomethane, which converted carboxylic acids to methyl esters for the quantification of PAH 12 .A second derivatization was also performed using N,O-bis-(trimethylsilyl)trifluoroacetamide and 1% Trimethylchlorosilane, which silylated hydroxyl substituents and allowed levoglucosan quantification 13 .Moreover, additional information on sample processing (e.g., handling, extraction methods, digestion) and details of the analytical procedures used in GC-MS were provided in previous studies [12][13][14] .Furthermore, the measurement of inorganic ion content was conducted through ion chromatography (IC), a method involving the extraction of particles into ultrapure deionized water using sonication, followed by filtration of the resultant solution and subsequent quantification of ion concentrations, as detailed in previous studies 15,16 .The inductively coupled plasma mass spectroscopy (ICP-MS) was employed to determine the concentrations of metals and trace elements, involving a hot block acid digestion process to extract particles from the filter into an acidic solution containing nitric (HNO3), hydrochloric (HCl), and hydrofluoric (HF) acids, as elaborated in previous studies 17,18 .After digestion, the samples were diluted with deionized water, aerosolized, and then introduced into the ICP-MS instrument (Thermo Finnigan Element2, Thermo Fisher Scientific Inc., Waltham, MA, USA).
Furthermore, DRI's Environmental Analysis Facility (EAF) operations adhere to stringent quality control and assurance protocols 19 , and the facility is accredited by the Texas Commission on Environmental Quality (TCEQ) through the National Environmental Laboratory Accreditation Program (NELAP) 20,21 , and US Environmental Protection Agency (EPA) [22][23][24] .
The DTT analysis used high-purity reagents with stringent storage and handling protocols.This analysis, conducted with an automated system, included thorough calibration and validation 25 .
The reaction vial (RV) was placed inside a thermomixer (550 rpm, 37 0 C) and then mixed with 5,5'-dithiobis-2-nitrobenzoinc acid (DTNB), followed by dilution with DI water in conical centrifuge vial (measurement vial, MV) 26 .DTT was quantified by measuring the absorption of the diluted MV mixture using a Liquid Waveguide Capillary Cell (LWCC-M-100; World Precision Instruments, Sarasota, FL, USA) connected to a spectrophotometer (Ocean Optics, Orlando, FL, USA) containing an ultraviolet-visible light source (Ocean Optics, Orlando, FL, USA).It should be noted for data acquired from laboratory analyses of chemical species and DTT assay, averages and standard deviations of field blank measurements were calculated and subtracted from measurements of chemical species and oxidative potential.
All calculations, statistical and analytical methods, including regression models and data visualizations, were performed using Python.

Table S1 .
Variation (Mean ± SD) of meteorological data for different periods.